1. Field of the Invention
The present invention relates to a color image forming apparatus and a control method thereof, and more particularly, to a color image forming apparatus and a control method thereof that include multiple color developing means and means for sequentially transferring images having multiple colors formed by the respective developing means.
2. Description of the Related Art
Electrophotography is known as an image recording method used in a color image forming apparatus such as a color printer or a color copy machine. The electrophotography method forms a latent image on a photosensitive drum using a laser beam and develops the image with charged color material (hereinafter, referred to as toner). Image recording is performed by transferring and fixing an image of the developed toner onto a transfer paper or media.
There has been an increase recently in the use of a tandem method for color image formation apparatus. The tandem method includes use of developing devices and photosensitive drums for each of the toner colors. Different color images are sequentially transferred onto an image transfer belt or a recording medium, in order to improve the image forming speed of the color image forming apparatus. This type of color image forming apparatus using the tandem method is known to have a plurality of factors that cause registration shifts. Various countermeasures have been proposed for each of the factors.
One of the factors is in-homogeneity or an attachment position shift of a lens in a deflection-scan device and a mount position shift of the deflection-scan device against a main body of the color image forming apparatus. These position shifts cause an inclination or a curve of a scanning line, and the degree of the curve (hereinafter, referred to as profile) is different for each color, thereby causing registration shifts.
The profile has different characteristics among image forming apparatuses, that is, among recording engines, and further among colors. Examples of the profiles are indicated by Reference numerals 1601 to 1604 in FIG. 16. In the profiles 1601 to 1604, the horizontal axis shows a position in a main-scan direction in the image forming apparatus. A line O expressed as a straight line along the main-scan direction shows an ideal characteristic without a curve. Also, line A, line B, line C, and line D expressed by curves show profiles for respective colors. The characteristics for cyan (hereinafter, designated by C), magenta (hereinafter, designated by M), yellow (hereinafter, designated by Y) and black (hereinafter, designated by K) are shown by the lines A, B, C, and D, respectively. The vertical axis shows an amount of shift in a sub-scan direction against the ideal characteristic. As apparent in the drawing, inflection points of the curves are different among the colors depending on the above mentioned factors and these differences appear to be the registration shits in image data after fixing of the toner.
As a countermeasure for these registration shits, Japanese Patent Laid-Open No. 2002-116394 discloses a method which measures the magnitude of a curve in the scanning line in an assembly process of a deflection-scan device using an optical sensor and fixes a lens with adhesive after adjusting the curve of the scanning line by rotating the lens mechanically.
Japanese Patent Laid-Open No. 2003-241131 discloses a method which measures the magnitude of a inclination of the scanning line using an optical sensor in a process mounting a deflection-scan device to a main body of a color image forming apparatus, adjusts the inclination of the scanning line by inclining the deflection-scan device mechanically, and thereby mounts the device to the main body of the color image forming apparatus.
Further, Japanese Patent Laid-Open No. 2004-170755 discloses a method which measures the magnitude of an inclination and curve in a scanning line using an optical sensor, corrects bit map image data so as to compensate the inclination and curve, and forms the corrected image. This method performs the correction electrically by processing the image data and thereby does not need a mechanical adjustment member or an adjustment process during assembly. Accordingly, it is possible to downsize the color image forming apparatus and to cope with the registration shift problem less expensively than the methods disclosed by Japanese Patent Laid-Open No. 2002-116394 and Japanese Patent Laid-Open No. 2003-241131. This electrical registration shift correction is classified into a correction by one pixel and a correction less than one pixel. The correction by one pixel offsets pixels by one pixel in the sub-scan direction according to the amount of correction of the inclination and curve as indicated by Reference numerals 1701 to 1703 in FIG. 17. Note that, in the description hereinafter, a position for the offset is referred to as a scan line changing pointscan line changing point. That is, in the diagram indicated by Reference numeral 1701 in FIG. 17 which shows background art, P1 to P5 correspond to the scan line changing pointscan line changing points.
The correction of less than one pixel (hereinafter, also referred to as interpolation processing) adjusts a gradation value of bit map image data in a preceding pixel or a following pixel in the sub-scan direction, as shown in the diagram indicated by Reference numerals 1801 to 1805 in FIG. 18 which show background art. That is, when the scanning line curves upward in the profile as shown in the diagram indicated by Reference numeral 1701 in FIG. 17 which shows background art, the bit map image data before gradation correction is corrected toward the reverse direction of the profile in the sub-scan direction. By performing the interpolation processing in this manner, it is possible to eliminate an unnatural step caused by the correction by one pixel at the scan line changing pointscan line changing point boundary and to smooth the image.
Additionally, there are two different cases in which this interpolation processing is to be applied and not applied, depending on the kind of image. For, example, it is preferable not to perform the interpolation processing on a pseudo-halftone image (hereinafter, referred to as screen image) using a dither matrix, because there is a possibility that intensity unevenness occurs to degrade image quality. Even for the screen image, the interpolation processing is preferably performed on an edge potion of the image to eliminate a jaggy image. Further, it is preferable to perform the interpolation processing in order to improve visibility of information by smoothing for a repeated pattern of the same design or print (hereinafter, referred to as pattern image), a character, a fine line, or the like, which can be drawn by office document creation software.
In this manner, while the correction by one pixel in the sub-scan direction is effective for reducing the registration shift, there is a case in which a step caused at the scan line changing pointscan line changing point becomes highly visible. A correction less than one pixel in the sub-scan direction, that is, the interpolation processing, is effective to reduce the effect of this step, but there is a case where the interpolation processing is not suitable depending on the kind of image data.
So as not to perform the interpolation processing for the unsuitable case, it is necessary to judge whether or not to perform the interpolation processing in the interpolation process of the conventional art. This judgment requires a determination of the kind of image data. However, there has been a problem that a circuit size is increased by a requirement of a large scale pattern matching processing or the like in this image determination for identifying the kind of image data. That is, the kind (characteristic) of image data should be determined for determining whether or not the interpolation processing is appropriate, that is, whether or not to perform the interpolation processing. For this purpose, a number of items are required to be performed such as screen determination, pattern determination, copy-forgery-inhibited pattern determination, false determination correction, etc., and further, the above plurality of determinations need to be performed on data for each color when the image data is composed of multiple colors. Accordingly, the circuit size becomes very large.